I usually estimate how much run (horizontal length) I'll need for each 1/2" of rise when I an track planning. I work in N-scale, where engines are a little more tolerant of grades (at least until, like me, you add weight to the cars), so for a tight condition I allow a 3% grade. If X is the variable that represents the run length, then 1/2" divided by X is going to be equal to 3 divided by 100. Rearranging things according to the rules of algebra, X = 1/2" times 100 divided by 3, so X = 16.5" roughly. Knowing this, I can cut my vertical risers to 1/2" increments, and place one every 16.5 inches along the line. Or, as I'm drawing the track plan, I just note the height of the track every 16.5 inches, whatever...
Of course, to achieve a more realistic 2% or less grade you'd solve for X the same way. At 2%, X = 1/2" times 100 divided by 2, so X = 25". If you don't mind fudging it a bit, you might just say that a 3% grade is 1/2" rise in 16" run, while 2% is 1/2" in 24", and 1% is 1/2" in 48".
Why do I use 1/2" of rise? Mostly because I am in N-scale and need it finer-tuned than just even inches, but also because 16" to 24" is a decent spacing between risers (some may prefer more risers placed closer, just depends on how you build your benchwork).